Many studies, in bacteria as well as in other microbial organisms, have shown that some of the repair-deficient strains are more mutable than the wild type. In studies with the ad-3 test system of Neurospora crassa, F.J. de Serres and his co-workers showed that repair-deficient strains, upr-1 and uvs-2, are more mutable than wild type after UV or gamma-ray irradiations. It has also been shown by de Serres that the spectra of genetic alterations caused by UV in repair-deficient strains are different from that caused by UV in the wild type. It is of interest to study and compare the mutagenic sensitivity of repair-deficient and wild-type strains to various chemical carcinogens and to compare the spectra of genetic alterations caused by these carcinogenic agents in the UV-sensitive and the wild-type strains.